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Abstract

Constituting of chemically cross-linked polymers, hydrogels are an interesting class of materials that are able to undergo significantly large deformation through the imbibement of solvent. These large deformations can also be triggered by external stimuli through appropriate change of constituents. In this thesis, the mechanical, thermodynamic and kinetic properties of various environmentally sensitive hydrogels were modeled and analyzed to study the different interesting phenomena exhibited, namely the phase transition and instability during swelling.
The thesis begins the study with investigation of the kinetics of neutral, temperature-sensitive, as well as pH-sensitive hydrogels. The swelling kinetics of these hydrogels were shown to closely resemble heat transfer laws, and finite element procedures on Abaqus were thus developed to simulate transient swelling behavior exploiting the close similarity in differential form between swelling kinetics and heat transfer laws. The finite element procedure was verified by comparisons with both analytical solutions and available experimental data. Several numerical examples of transient swelling behavior of hydrogels, including inhomogeneous free swelling of a cube, constrained swelling of a gel blanket, three-dimensional swelling of a gel block, and interactions with hard and soft surfaces, were subsequently investigated.
Next, the phenomena of phase transition and large deformation of temperature-sensitive, photo-thermo-sensitive and magneto-thermo-sensitive hydrogels were investigated. This was done by considering the chemical thermodynamics of the gels in equil4ibrium with external loads and environmental stimuli. Constitutive equations were derived and the material models were implemented on Abaqus using both UHYPER and UMAT subroutines. It was shown that the phase transition temperature of a temperature sensitive hydrogel could be altered by the addition of foreign particles, and also through the variation of strength of external stimuli. Several numerical
examples highlighting interesting phenomena, such as phase transition and phase co-existence, and potential applications in microfluidic devices were subsequently studied.
Lastly, this thesis examines the instabilities induced when a gel swells. Various cases of buckling, including buckling of a thin annulus, bifurcation of a gel layer with periodic hole array, transient surface wrinkling during swelling were studied. The analysis was carried out using finite element models developed in the earlier studies and also through linear perturbation analysis for the case of surface wrinkling.